1. Field of the Invention
The present invention relates to a dispersion compensator for compensating for the chromatic dispersion and dispersion slope of an optical fiber transmission line; and an optical transmission system comprising an optical fiber transmission line and a dispersion compensator.
2. Related Background Art
In optical transmission systems carrying out optical communications by propagating signal light through an optical fiber transmission line, it is desired that the accumulated chromatic dispersion of the optical fiber transmission line have a small absolute value at the signal light wavelength (e.g., 1.55 xcexcm) in order to suppress the waveform deterioration of the signal light when it propagates through the optical fiber transmission line. Also, in wavelength division multiplexing (WDM) transmission systems carrying out optical communications by multiplexing a plurality of wavelengths of signal light, it is desired that the accumulated chromatic dispersion of the optical fiber transmission line should be small in a signal light wavelength band including individual wavelengths of the plurality of wavelengths of signal light. Namely, it is desirable for the optical fiber transmission line to have not only a small absolute value of chromatic dispersion but also a small absolute value of dispersion slope in the signal light wavelength band.
However, typical single-mode optical fibers employed as optical fiber transmission lines in general have a zero-dispersion wave length near a wavelength of 1.3 xcexcm, while having a chromatic dispersion of about 17 ps/nm/km and a dispersion slope of about 0.058 ps/nm2/km at a wavelength of 1.55 xcexcm. As a consequence, it is difficult to carry out high capacity WDM transmission if an optical fiber transmission line is constituted by these single-mode optical fibers alone.
Therefore, it has been proposed to compensate for the chromatic dispersion and dispersion slope of a single-mode optical fiber by using a dispersion-compensating optical fiber whose chromatic dispersion and dispersion slope are both negative at a wavelength of 1.55 xcexcm, so as to lower both the respective absolute values of the average chromatic dispersion and average dispersion slope of the whole system at a wavelength of 1.55 xcexcm.
For compensating for both the respective absolute values of the average chromatic dispersion and average dispersion slope of the whole system comprising a single-mode optical fiber and a dispersion-compensating optical fiber, it is necessary that both the chromatic dispersion and dispersion slope of the dispersion-compensating optical fiber should be designed appropriately according to the length ratio between the single-mode optical fiber and the dispersion-compensating optical fiber and the chromatic dispersion characteristic of the single-mode optical fiber, and that the dispersion-compensating optical fiber be manufactured as designed.
However, the chromatic dispersion characteristic of the dispersion-compensating optical fiber is quite sensitive to changes in the drawing tension and core diameter in the longitudinal direction at the time when its optical fiber preform is drawn. Also, the processing precision of the optical fiber preform may not be sufficient, and the accuracy in measurement of refractive index distribution in the optical fiber preform carried out by a preform analyzer is may not be sufficient, either. Therefore, it is difficult for a dispersion-compensating optical fiber having the aimed chromatic dispersion characteristic to be manufactured with a high precision.
For example, while the dispersion slope compensation ratio xcex7 is ideally desired to be 100%, the dispersion slope compensation ratio xcex7 may fluctuate among actually manufactured dispersion-compensating optical fibers approximately within the range of 50% to 120% with its average value being about 90%. Here, the dispersion slope compensation ratio xcex7 is an index representing the degree of compensation when a dispersion-compensating optical fiber compensates for the chromatic dispersion and dispersion slope of a single-mode optical fiber. The dispersion slope compensation ratio xcex7 (%) is defined by the following expression:
xcex7=100xc2x7(DSMF/SSMF)/(DDCF/SDCF)xe2x80x83xe2x80x83(1) 
where DSMF is the chromatic dispersion of the single-mode optical fiber, SSMF is the dispersion slope of the single-mode optical fiber, DDCF is the chromatic dispersion of the dispersion-compensating optical fiber, and SDCF is the dispersion slope of the dispersion-compensating optical fiber.
Thus, if the dispersion slope compensation ratio xcex7 of an actually manufactured dispersion-compensating optical fiber greatly differs from the ideal value of 100%, then this dispersion-compensating optical fiber cannot fully compensate for the chromatic dispersion and dispersion slope of the single-mode optical fiber. In this case, it is hard to carry out WDM transmission over a wide band and optical transmission with a high bit rate (e.g., 40 Gb/s).
Here, the dispersion slope compensation ratio xcex7 of each of manufactured dispersion-compensating optical fibers may be measured, so as to selectively use those whose dispersion slope compensation ratio xcex7 lies within a predetermined range. However, this, procedure is undesirable in that the yield is not favorable, so that the price of conforming products becomes higher
In order to eliminate the above-mentioned problems, it is an object of the present invention to provide a dispersion compensator which can fully compensate for the chromatic dispersion and dispersion slope of an optical fiber transmission line even when the dispersion slope compensation ratio xcex7 fluctuates among actually manufactured dispersion-compensating optical fibers, and an optical transmission system in which both the respective absolute values of the average chromatic dispersion and average dispersion slope of the whole system are lowered.
The dispersion compensator in accordance with the present invention is a dispersion compensator for compensating for the chromatic dispersion and dispersion slope of an optical fiber transmission line in which a plurality of dispersion-compensating optical fibers each having a dispersion slope compensation ratio of at least 60% with respect to the optical fiber transmission line at a predetermined wavelength (e.g., 1.55 xcexcm) are connected to each other, one of the plurality of dispersion-compensating optical fibers has a dispersion slope compensation ratio of at least 80% and another of the plurality of dispersion-compensating optical fibers has a dispersion slope compensation ratio within the range of 60% to 100%.
The average dispersion slope compensation ratio of this dispersion compensator is an average value of the respective dispersion slope compensation ratios of the plurality of dispersion-compensating optical fibers in a certain sense, and can become a value near 100% if the ratio of respective lengths of the plurality of dispersion-compensating optical fibers is appropriately set. Hence, if the respective lengths of the plurality of dispersion-compensating optical fibers are appropriately set according to the length of an optical fiber transmission line (a single-mode optical fiber in general), then the dispersion compensator can fully compensate for both the chromatic dispersion and dispersion slope of the optical fiber transmission line. Also, even when the dispersion slope compensation ratio xcex7 fluctuates among actually manufactured dispersion-compensating optical fibers, the dispersion slope compensation ratio xcex7 of each dispersion-compensating optical fiber is measured after manufacture, and a plurality of dispersion-compensating optical fibers are connected according to results of the measurement so as to construct the dispersion compensator as mentioned above. As a consequence, the manufactured dispersion-compensating optical fibers can be used efficiently, whereby the dispersion compensator becomes inexpensive.
In the dispersion compensator in accordance with the present invention, the average dispersion slope compensation ratio of the plurality of dispersion-compensating optical fibers may be at least 80%. In this case, the dispersion compensator can fully compensate for both the chromatic dispersion and dispersion slope of the optical fiber transmission line.
In the dispersion compensator in accordance with the present invention, a plurality of dispersion-compensating optical fibers may be connected in the order of increasing effective area thereof. In this case, signal light having a higher power propagates through a dispersion-compensating optical fiber having a relatively larger effective area, so that the generation of nonlinear optical phenomena can be suppressed, whereby transmission characteristics become excellent.
In the dispersion compensator in accordance with the present invention, of the plurality of dispersion-compensating optical fibers, first and second dispersion-compensating optical fibers cascaded to each other may be fusion-spliced to each other. In this case, the splice loss between the first and second dispersion-compensating optical fibers is low. In this case, it is preferred that the fusion-spliced portion between the first and second dispersion-compensating optical fibers be re-coated with a resin material having a coating diameter substantially equal to the coating diameter of each of the first and second dispersion-compensating optical fibers, and be wound about a bobbin together with the first and second dispersion-compensating optical fibers. Also, it is preferred that the fusion-spliced portion between the first and second dispersion-compensating optical fibers be re-coated with a resin material and be secured with reinforcement means. It is also preferred that the plurality of dispersion-compensating optical fibers be contained in a bundle state free of any body portion substantially in contact therewith. In any of these cases, each of the plurality of dispersion-compensating optical fibers can be contained in a compact state while securing a mechanical strength in the fusion-spliced portion and suppressing the increase in transmission loss.
The optical transmission system in accordance with the present invention comprises an optical fiber transmission line for transmitting signal light; and the above-mentioned dispersion compensator, in which a plurality of dispersion-compensating optical fibers are connected to each other, for compensating for the chromatic dispersion and dispersion slope of the optical fiber transmission line. In this optical transmission system, the dispersion compensator compensates for the chromatic dispersion and dispersion slope of the optical fiber transmission line, whereby the respective absolute values of the average chromatic dispersion and average dispersion slope of the optical fiber transmission line and dispersion compensator are lowered. As a consequence, this optical transmission system can carry out WDM transmission over a wide band and optical transmission with a high bit rate.
The optical transmission system in accordance with the present invention may be configured such that a plurality of dispersion-compensating optical fibers are connected in the order of increasing effective area thereof, and that signal light is fed from the side of the dispersion-compensating optical fiber having a larger effective area. In this case, the occurrence of nonlinear optical phenomena can be suppressed in the dispersion compensator, whereby transmission characteristics become excellent.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.